Recovery of gaseous and liquid hydrocarbons from petroleum reservoirs often requires drilling of boreholes into the formations in which the petroleum reservoirs are to be found. Some petroleum reservoirs are in subsea formations. In some instances deviated or lateral boreholes are required to access the petroleum reservoirs. Deep boreholes may pass through multiple strata of complex and differing geological characteristics before penetrating the formation of interest.
A risk frequently encountered in any such boreholes is one of bore wall collapse due to instability in the surrounding formation. This is addressed in the industry by introducing tubulars such as casing and liner sections which are cemented into place after positioning. As the bore depth increases, successively narrower tubulars may be positioned in the bore and the overlap zone between the tubulars may be at least about 30 meters which also requires cementing.
It is important that the cement surrounds the introduced tubular to secure and seal it in position. Therefore it is also important to ensure an adequate clearance between the introduced tubular and the bore wall or previously installed casing tubular to admit the appropriate amount of cement and allow proper distribution around the introduced tubular.
Use of centralisers has been proposed as an approach to position tubulars centrally within the bore. These centralisers are of generally cylindrical shape and may comprise rigid bodies, or bow spring elements positioned between end collars to be slipped over a tubular. Alternatively the centraliser may be of a hinged segmented type with lockable end bands to encircle the tubular.
Difficulties with centralisers may be encountered where restrictions in the bore diameter have to be traversed, in which case the centraliser may not pass the restriction. Alternatively, a centraliser may have insufficient width when introducing a tubular through a bore or casing of one width dimension to one where the bore is widened such as for example in an underreamed well zone.